This invention relates to an antenna unit for use in receiving an electric wave of a digital radio broadcasting and a receiving system therefore.
In recent years, a digital radio receiver, which receives a satellite wave or a terrestrial wave to listen the digital radio broadcasting, has been developed and is put to practical use in the United State of America. The digital radio receiver is mounted on a mobile station such as an automobile and can receive an electric wave having a frequency of about 2.3 gigahertz (GHz) to listen in a radio broadcasting. That is, the digital radio receiver is a radio receiver which can listen in a mobile broadcasting. Inasmuch as the received wave has the frequency of about 2.3 GHz, a reception wavelength (resonance wavelength) λ thereof is equal to about 128.3 mm. In addition, the terrestrial wave is an electric wave in which a signal where the satellite wave is received in an earth station is frequency shifted a little.
Inasmuch as the electric wave having the frequency of about 2.3 GHz is used in the digital radio broadcasting in the manner which is described above, it is necessary to set up an antenna outside the automobile. Accordingly, the antenna must be attached to a roof of the automobile in a case where the digital radio receiver is mounted in the automobile.
Such as an antenna for the automobile and a receiver body (a head unit) put inside a room of the automobile are electrically connected to each other through a cable. The head unit serves as an external device.
A receiving system of the type described for receiving the satellite wave of the terrestrial wave includes an antenna portion (an antenna unit) and a tuner portion (a satellite radio tuner). The antenna portion (the antenna unit) receives the satellite wave or the terrestrial wave to produce a received signal having a high frequency. The tuner portion (the satellite radio tuner) demodulates the received signal into a speech signal.
In conventional receiving systems, the antenna portion (the antenna unit) and the tuner portion (the satellite radio tuner) are constructed in a separated state each other. In other words, a conventional antenna unit comprises an antenna element for receiving the satellite wave or the terrestrial wave and a low noise amplifier (LNA) for amplifying an electric wave received by the antenna element to produce a received signal.
On the other hand, head units are classified into a tuner-equipped head unit equipped with a satellite radio tuner and a tuner-unequipped head unit equipped with no satellite radio tuner.
First and second conventional receiving systems are known in the art. The first conventional receiving system uses, as the head unit, the tuner-equipped head unit while the second conventional receiving unit uses, as the head unit, the tuner-unequipped head unit.
The first conventional receiving system comprises an antenna unit and the tuner-equipped head unit The antenna unit and the tuner-equipped head unit are connected to each other through an RF coaxial cable. The antenna unit comprises an antenna element and a low noise amplifier (LNA). The tuner-equipped head unit is equipped with a satellite radio tuner.
The second conventional receiving system comprises an antenna unit, the tuner-unequipped head unit, and a satellite radio receiver. The antenna unit and the satellite radio receiver are connected to each other through an RF coaxial cable. The satellite radio receiver and the tuner-unequipped head unit are connected to each other through a bus cable. The satellite radio receiver is equipped with a satellite radio tuner.
In addition, as a conventional antenna unit, a planar antenna may be used which is disclosed in Japanese Unexamined Patent Application Publication Tokkai No. 2001-24428 or JP-A2001-24428. According to JP-A 2001-24428, the planar antenna comprises an upper case, a planar antenna element disposed at backside of the upper case with regular intervals, a ground plate disposed at regular intervals from the planer antenna element, feeding leads disposed between the planer antenna element and the ground plate, a circuit board connected to a lower surface of the ground plate, and electronic parts mounted on the circuit board. The electronic parts include the above-mentioned low noise amplifier (LNA).
In the first conventional receiving system, in spite of an audience contract for the satellite radio broadcasting, it is necessary for a set maker to equip the head unit (the external device) with the satellite radio tuner on making a plan for a satellite radio tuner adapted model.
On the other hand, in the second conventional receiving system, the satellite radio receiver equipped with the satellite radio tuner is required in addition to the head unit (the external device) and further the bus cable for connecting between the head unit and the satellite radio receiver is required.
In addition, inasmuch as the RF coaxial cable has a gain loss, the RF coaxial cable is disadvantageous in that it is impossible to lengthen a cable's length thereof.